76 



MORPHOLOGY OF THE CELL. 



such as caustic potash, and the like. But sometimes the cell- 

 wall seems to be completely changed into cork-substance. 



249. Cork-substance behaves towards reagents in nearly all 

 respects as cutin does (see 157). 



250. Cells which have been completely suberized can be sepa- 

 rated from each other by the gradual action of Schulze's macer- 

 ating solution. 1 



251. The color of cork-cells is not dependent upon the amount 

 of the change of the wall into cork-substance. The walls of the 

 cells in some species of willow are colorless, while those in other 

 species are distinctly yellow ; and yet the former have been as 

 thoroughly changed into cork-substance as the latter. 



II. 



Cells of the Fibro-vascular System, Proseiichyma in the 

 widest sense. 



252. The cells and modified cells of this system constitute 

 the framework of a plant. In a few of the higher and in many 

 of the lower plants it is barely if at all developed, the entire 

 structure consisting, in such cases, of a mass of parenchyma 

 covered by epidermis. But in most plants it exists as a skeleton 



1 This fact has led to the belief that there exists in such cases an interme- 

 diate plate which differs in its character from the rest of the cell-wall ; but 

 prolonged action of the same reagent, especially with warming, causes the cells 

 to break down and ultimately form a disorganized mass. 



Fio. 57. Formation of cork and secondary cortex in Betulaverrucosa. A, B, C, D t 

 nccettsiro Htage*; 1, tlrnt layor of secondary cortex; 2, layer which divides In /*, to give 

 CHUM ! tin- lirst layer of curk (shown in C), and a layer, 3, within, which again divide* 

 in/'. (Sauio.) 



